The present invention relates to seismic surveying. In particular, it relates to a method of and system for seismic surveying which makes use of information about uncertainty in the signature of a seismic source.
The principle of seismic surveying is that a source of seismic energy is caused to emit seismic energy such that it propagates downwardly through the earth. The downwardly-propagating seismic energy is reflected by one or more geological structures within the earth that act as partial reflectors of seismic energy. The reflected seismic energy is detected by one or more sensors (generally referred to as “receivers”). It is possible to obtain information about the geological structure of the earth from seismic energy that undergoes reflection within the earth and is subsequently acquired at the receivers.
When a seismic source array is actuated to emit seismic energy it emits seismic energy over a defined period of time. The emitted seismic energy from a seismic source array is not at a single (temporal) frequency but contains components over a range of frequencies. The amplitude of the emitted seismic energy is not constant over the emitted frequency range, but is frequency dependent. The emitted seismic energy from a seismic source array may also vary in space due to two factors: the source array may emit different amounts of energy in different directions, and the seismic wavefronts may “expand” with time (expanding spherical waves as opposed to plane waves). The seismic wavefield emitted by a seismic source array is known as the “signature” of the source array. When seismic data are processed, knowledge of the signature of the seismic source array used is desirable, since this allows more accurate identification of events in the seismic data that arise from geological structures within the earth. In simple mathematical terms, the seismic wavefield acquired at a receiver is the convolution operation of two factors; one representative of the earth's structure, and another representative of the wavefield emitted by the source array. The more accurate is the knowledge of the source array's signature, the more accurately the earth model may be recovered from the acquired seismic data.
A manufacturer of a seismic source may provide a general source signature for the seismic source. However, each time that a seismic source is actuated the actual emitted wavefield may vary slightly from the theoretical source signature. In a typical seismic survey a seismic source array is actuated repeatedly and seismic data are acquired consequent to each actuation of the source array. Each actuation of the source array is known as a “shot”. In processing seismic data it is desirable to know as accurately as possible the source signature for each shot.
It has been suggested that one or more seismic receivers may be positioned close to a seismic source, in order to record the source signature. By positioning the seismic receiver(s) close to the seismic source the wavefield acquired by the seismic receiver(s) should be a reliable measurement of the emitted source wavefield. WesternGeco's Trisor/CMS system provides estimates of the source wavefield from measurements with near-field hydrophones near each of the seismic sources composing the source arrays in marine seismic surveys. These estimates have been used to control the quality and repeatability of the emitted signals, and to perform compensation for shot-to-shot variations or source-array directivity. Recent comparison of signals, predicted by the Trisor/CMS system or recorded with point-receiver hydrophones (Q-marine system), indicate that the quality of the Trisor/CMS estimates is excellent over a large band of frequencies and source take-off angles.
FIG. 1 shows a comparison between a Trisor/CMS predicted incident wavefield (a) and an incident wavefield measured with a near-offset hydrophone on a Q-marine streamer, towed 23 m deep (b). The waveforms have been bandlimited to a range of frequencies between 1 and 120 Hz. It can be seen that the agreement between the two waveforms is very good over this range of frequencies. Note that the energy is propagating to the near-offset hydrophone following a nearly horizontal raypath corresponding to a take-off angle of 80 degrees.
The Trisor/CMS incident wavefield is the result of a computation involving several measurements or estimated quantities and some assumptions, as described for instance in Ziolkowski, A. et al., “The signature of an air gun array: Computation from near-field measurements including interactions”, Geophysics, 47, No. 10, p. 1413-1421 (1982).
The key factors influencing the estimation are the position data for the guns and near-field hydrophones, as well as the estimate of the free surface reflection coefficient.